Injection systems and methods heretofore have been devised for blending one or more additives into a main fluid flow stream according to a controlled ratio. Typical additives include fuel additives, catalysts, dyes, lubricants, water treatment chemicals, etc. and typical areas of application include gasoline truck loading terminals, marine terminals, chemical processing plants, water treatment facilities, etc.
Additive injection equipment oftentimes is located in hazardous areas such as fu, el loading terminals where explosive vapors may be present. This usually requires mounting the electrical portions of the equipment inside explosion-proof enclosures. In some fuel additive injection systems, an electronic controller is contained in an explosionproof housing. This presented a problem in that the enclosure would have to be opened to gain access to the therein contained electronic equipment in order to locally change additive rates, adjust meter factors, etc. Before opening the enclosure, rigorous care would be taken to ensure that no hazardous vapors are present, all electrical power is turned off, proper safety practices are followed, etc., and this may require the assistance of other personnel such as a certified union electrician. In another known additive injection system, an external key pad has been provided to perform operations that previously required the opening of the enclosure. However, a key pad is subject to mechanical failure especially when exposed to harsh environmental conditions that may be encountered in a fuel loading terminal or other installation.
In fuel loading terminals genetic gasoline delivered by a pipeline to the terminal in many instances is sold to different gasoline marketing companies. The genetic gasoline becomes the proprietary products of the different marketing companies when their particular additive is blended into the genetic gasoline. Such blending typically takes place when the gasoline is being loaded into a tanker truck for further distribution. Additive injection equipment is used to blend the additive into the gasoline flow at a controlled ratio.
Additive injection systems heretofore used in fuel loading terminals have employed an additive injector including a solenoid valve to control the flow of additive through an additive flow line leading to the main fuel line through which fuel is dispensed from a storage tank to a truck being loaded. On start up of fuel flow, an external switch or automation system sends a permissive signal to the controller for the additive injector to enable the operation of additive feed at a selected ratio. Additive is cyclically injected into the main fuel line in prescribed doses at a rate determined by the rate of flow of fuel through the main fuel line. That is, for each preset quantity, such as 40 gallons (151.4 liters), of fuel that flows through the main fuel line as measured by a flow meter, a prescribed amount or dose, such as 100 cc, of additive is injected into the main fuel line. In this manner the additive is blended into the entire load of fuel ,at a selected ratio.
When additive is being injected, additive flow is measured by a flow meter and the additive flow control valve remains open until the dose amount has been injected. The additive flow control valve then closes stopping additive flow temporarily and until a next dose injection cycle is initiated, at which time the additive control valve is opened once again. Because it takes any solenoid valve a finite amount of time to close, a small amount of additive will flow through the control valve as it is closing. This small amount of additive should be taken into account and corrected for in subsequent injection cycles in order to obtain precise control over the total amount of additive injected into the main stream flow. According to a known correction technique, the next injection dose would be adjusted by any overage (or underage) of a preceding injection dose. That is, if "x" is the desired injection dose and the additive flow meter measured an amount of additive injected during the preceding dose as "x+a", then the amount of additive to be injected in the next cycle would be set at "x-a". In this manner, any overage would be accounted for if "a" were a positive number and any underage would be accounted for if "a" were a negative number.
Although in theory the foregoing correction technique would appear to provide for accurate correction of any overage or underage associated with the finite time required for the solenoid valve to close, the technique is subject to an accumulation of rounding errors. When the amount of overage or underage "a" is rounded to the nearest incremental mount that the system controller is capable of handling, the actual correction will deviate from the needed correction by the rounding error. For a total load of product, the accumulation of these rounding errors may give rise to a substantial variance between the amount of additive sought to be injected and the amount of additive that actually is injected during the loading operation.
A need exists in many applications for obtaining more precise control over the amount of additive injected than that afforded by the foregoing correction technique. In the case of fuel loading terminals, accurate control over the amount of additive injected into the fuel is desired to ensure the integrity of the ultimate product as well as to prevent over injection of high cost fuel additives.